This invention relates generally to self-purging ethylene vinyl alcohol (EVOH) compositions. While the principles of the invention can apply to a wide variety of self-purging EVOH compositions, the invention is described herein specifically with is respect to those self-purging compositions containing metal salts of acids. Accordingly, the principles described herein are not limited to polymer compositions containing metal salts, but are limited only by the operability of the principles disclosed herein with respect to odor and flavor suppression properties attendant the materials disclosed herein for controlling such properties.
EVOH resin is widely employed for, among other things and without limitation, food packaging and other applications as a gas and/or flavor barrier material. EVOH is usually extruded from a die in a single or multiple layer structure, and may thus be fabricated into a wide variety of shapes, such as films, sheets, or bottles.
Melted EVOH resin has a known affinity for metals, and thus tends to build up inside the extruder and downstream equipment (e.g. adapter, screens, and shaping die) of related polymer processing apparatus. EVOH is also quite sensitive to the time during which it is exposed to extruder-type temperatures. Namely, the "OH" moieties in EVOH tend to cross-link with each other.
The physical result is that, as a typical extrusion "run" progresses, viscosity of the EVOH extrudate gradually increases (MI decreases), while the amount of cross-linked EVOH builds up inside the extrusion and shaping equipment. As material continues to build up in the extrusion and shaping equipment, at some point the build-up is great enough that the material begins to break loose and show up in the extrudate as gels. The number of gels increases with time. Large numbers of gels are generally unacceptable in shaped products so made. Thus, the extruder must, from time-to-time, be purged (cleaned out) of the EVOH build-up.
One method for cleaning out the EVOH is to shut down, cool off, disassemble, and physically scrape/clean, etc. the extrusion equipment. A preferred method is to run through the extrusion and shaping equipment some material that will clean out the EVOH, including the build-up inside the respective equipment. Such material is generically known as a "purge" material, and the process is called "purging."
With EVOH, "purging" must be done periodically, even if the resin being extruded is always EVOH. Using conventional technology, the interval between "purgings" of EVOH is typically 1-7 days. The longer runs of up to 7 days require use of a self-purging EVOH resin. By self purging EVOH resin, we mean that the resin composition includes at least one component/additive which participates in a controlled amount of cleavage of the EVOH copolymer, normally by cleavage of the ethylene chain.
Typical such additives are metal salts of acids, for example, magnesium stearate. Such cleavage of the polymer chain can create short chain polymer segments, as well as oxygen-containing monomer molecules such as aldehydes and ketones, some of which have human-perceived odors and/or flavors.
This invention is directed toward improved self-purging EVOH copolymer compositions, methods of use of such self-purging copolymer compositions, and products made with such self-purging copolymer compositions.
It is an object of the invention to provide an EVOH copolymer composition providing improved control of odor and flavor.
It is a more specific object to provide a self-purging EVOH copolymer composition providing improved control of odor and flavor.
It is a further specific object to provide an EVOH copolymer composition comprising a phenol and/or bisphenol stabilizer.
It is a more specific object to provide an EVOH copolymer composition comprising a tocopherol stabilizer.
It is a still more specific object to provide an EVOH copolymer composition comprising an alpha tocopherol stabilizer.
It is yet another object to provide a method of effectively controlling odor and/or flavor in the extruded product.
It is still another object to provide a method of making an extended extrusion run of EVOH copolymer composition while effectively controlling odor and/or flavor in the extruded product.